The Monroe 2880 is pretty unique among
the Monroe electronic calculators that I've come across. First off, it
is a customizable calculator. The 2880, though it can serve as a
normal office calculator, can be customized with a plug-in ROM
module that allows the calculator to assume varying application
personalities. This particular machine came with a Payroll ROM module that
provides complete payroll processing functions. If anyone out there
knows of other application ROM packs that could be had for this machine,
I'd be interested in hearing from you. My guess is that Monroe made
a number of different ROM packs that provided things like statistics,
engineering/scientific, and financial (e.g., bond trading, mortgage calculations).

Monroe 2880 with Top Cover Removed

The Monroe 2880 was built in late 1979
to early 1980, based on date codes on the IC's within the machine. The
machine is a microprocessor-based system, with a CPU [Part numbers
in parenthesis] (11660), three ROM chips (varying part numbers, probably
based more on ROM content), two RAM chips (10932), and three GPIO
(General Purpose I/O) chips (10696). All of the LSI IC's in the machine
are made by Rockwell, and are all socketed "ZIP" style packages.
Three AMI-made 7931 RAM (Random Access Memory) chips provide USER RAM storage,
which appears to be RAM reserved for use by the ROM-pack applications, while
the CPU's RAM is for overhead like stack space, temporary storage, calculator
register storage, etc. A number of Texas Instruments 74LS-series and Motorola
21000-series small and medium-scale glue devices tie all of the large scale
devices together.
The master clock is generated by a 3.579545 MHz crystal.
The display is rather unique on this machine. It is a 20-character
vacuum-fluorescent integrated display module, using a 14-segment "starburst"
arrangement to form each digit or character.

Closer view of Main Circuit Board

Display Controller Circuit Board

The display subsystem is controlled by a fairly complex board that connects
to the main board via ribbon cable. The display controller board connects
to the display module itself with a number of ribbon cables. It appears
that some custom driver IC's are used to drive the VF display, as there
is no detectable manufacturer logo or date codes on the three identical
IC's that appear to provide this function. The starburst format of the
display allows the display to show alpha characters as well as numerics.
Each digit position also has decimal point/comma segments. When operating
as a calculator, the calculator automatically positions commas every three
digits in front of the decimal point for easier reading.

Display in 'Calculator' Mode

Display Showing Alpha Prompt

The 2880 provides the basic functionality
of a five function business calculator. It is a fixed-point machine, with
decimal point selections of 0 through 8 digits behind the decimal point. The
[SET DP] button on the keyboard is pressed, followed by a single digit
from 0 to 8 to select the decimal point position. The calculator comes
up by default with 2 digits behind the decimal point at power up.
Calculations are carried out to 12 digits, with overflow indicated by the
display resetting to zero. The calculator does not lock out the keyboard
on overflow.
Addition and subtraction are done adding-machine style, with an
accumulator that acts almost like a memory register. The [+] key
acts like a [M+] key on a more conventional calculator, and the [-]
key acts like [M-]. The [diamond] key recalls the current accumulated total
(akin to a [MR] key) , and the [*] key recalls the total and clears the
accumulator (like pressing [MR] followed by [MC] on a calculator
with memory). A LED on the keyboard panel lights to indicate when the
accumulator register has non-zero content. Multiplication and division work
algebraically, with the [=] key calculating the result. The [=+] and [=-] keys
calculate the result of a multiplication or division, and add/subtract the
result to/from the accumulator register. The [#] key forces (even if
the printer is shut off) the current number in the display to be printed
on the printer with a "#" in the left-most column for annotation or other
purposes. The [C/CE] key is a dual function key (though it seems it should have
been labeled "CE/C"), with one press clearing the
display, and the second press clearing the calculator (but not resetting
program functions). A [CHG SIGN] key toggles the sign of the current
number in the display. In addition to the basic four functions, the
2880 provides a percentage function.

Monroe 2880 Keyboard Detail

The 2880 has a printer that prints
22 columns on standard adding-machine paper. The printer is a line at a time
type, with a line of characters/digits all printed at once. Printing is
done by plastic wheels with digit and/or character reliefs on them,
which turn to the correct character and then the paper is driven towards
the wheels (with ribbon between the wheels and the paper) to print the
characters on the paper. The ribbon is a two-color
ribbon in a cartridge, allowing the printer to print negative numbers in
red. The printer has limited alphabetic printing capability (the right-most
4 or 5 columns, anyway) to allow annotation of printed data. The printer
can be turned on or off by a switch located on the keyboard panel.

Close up of Monroe 2880 CPU Chip

The ROM Module that personalizes the
2880 plugs into a slot at the top of the cabinet. If the ROM module is
not present, pressing any of the fourteen soft-keys that provide access to the
ROM functions causes the machine to 'TRAP', resulting in a diagnostic
code showing up on the display, and a cryptic dump of a couple of lines
on the printer (even if the printer is turned off). The ROM module
defines the function of two rows of seven soft-keys which, in the
case of the payroll module that came with the machine, allow entry of
things like hourly wage, tax withholding status, hours worked, overtime
hours, etc. The left side of the keyboard has a number of keys related
to the program module, including [F1], [F2], and [F3] function keys, a
key labeled [EXECUTE] that seems to start the payroll processing
program, a [RE-START] key that seems to be the master reset for
the machine, and a key with a brick-red colored square on it which
is a second function key for the soft-keys, allowing each soft-key to perform
two differing functions. A green LED located near the second function key
serves as a ready indicator. When the machine is busy performing calculations,
this indicator goes out, and re-lights when the operation is complete.
Some of the payroll calculating functions can churn for 3 to 4 seconds.

The Payroll Application ROM Cartridge

The 2880 runs off 110V AC power only, and uses a conventional
transformer-based, IC and transistor-regulated linear power
supply to generate the working voltages needed to operate the machine.

In calculator mode, which the machine comes up in by default
when powered up, the machine is very fast, with all operations taking
approximately the same period of time to complete. There seems to be
no difference in the time that it takes for the machine to perform difficult
problems (such as all 9's divided by 1) and simple problems.
Overflow and math errors (divide by zero) result in the display clearing
to zero, and the printer spitting out a line of decimal points (even when
disabled). The display is blanked during calculation, which results
in a barely perceptable flicker when doing regular math operations. The
payroll application can leave the display blanked for significant periods
of time while calculations are being performed. The ready indicator
is the only way the user can tell that the machine hasn't "gone off
into the weeds". The keyboard is buffered, so that when the printer
is running, the operator can be up to 8 keystrokes ahead of the
printer, and no keystrokes will be lost. If the operator gets too far ahead of
the printer, there is no lockout or other indication that keystrokes
have been discarded due to the keyboard buffer overflowing.